A Dinuclear MnIII–CuII Single‐Molecule Magnet

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Abstract
The reaction of 1/3 equivalent of CuCl2⋅2 H2O with MnCl2⋅4 H2O and 5-bromo-2-salicylideneamino-1-propanol (H25-Br-sap) in methanol gave dark brown crystals of [MnIIICuIICl(5-Br-sap)2(MeOH)] (1). Complex 1 has an alkoxo-bridged dinuclear core of MnIII and CuII ions, which have elongated octahedral and square-planar coordination geometries, respectively. In dc magnetic susceptibility measurements, χmT values increased as the temperature was lowered, followed by a sudden decrease below 20 K. This behavior is indicative of the occurrence of intramolecular ferromagnetic interactions, and fitting gave an S=5/2 spin ground state with an exchange coupling constant JMnCu of +78 cm−1. Magnetization data collected as a function of temperature and applied magnetic field were analyzed by using a spin Hamiltonian with isotropic Zeeman and axial zero-field splitting (ZFS) terms, and a negative D5/2 value (−1.86 cm−1) was obtained. A high-field EPR (HFEPR) spectrum (342.0 GHz) at 4.2 K was composed of four peaks, and two additional peaks at higher magnetic field appeared as the temperature was increased. The temperature dependences in the HFEPR spectra are indicative of a negative D5/2 value, and fitting of the data gave D5/2=−1.81 cm−1. In the ac magnetic susceptibility measurements, frequency dependent in-phase (χm′) and out-of-phase (χm′′) signals with peak maxima at 0.7–1.5 K were observed and small peaks below 0.7 K appeared. The ac susceptibility data supports that 1 is a single-molecule magnet (SMM). Arrhenius plots for the χm′′ peaks from 0.7–1.5 K gave the re-orientation energy barrier (ΔE) of 10.5 K with a pre-exponential factor of 8.2×10−8 s.

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